1. Field of the Invention
The present invention relates to data compression, and in particular relates to devices and methods of encoding image data.
2. Description of the Related Art
The rapid growth of digital imaging applications, including desktop publishing, multimedia, teleconferencing, and high-definition television (HDTV) has increased the need for efficient and standardized image compression techniques. Without image compression, the transmission of images would require an unacceptable bandwidth for many applications. As a result, methods of compressing images have been the subject of numerous research publications. Image compression schemes convert an image consisting of a two-dimensional array of pixels into a sequence of bits which are transmitted over a communication link. Each pixel represents the intensity of the image at a particular location therein. The transmission link may be an ordinary telephone line.
Consider an image comprising a gray-scale representation of a photograph at a resolution of 1000×1000 lines. Each pixel typically consists of 8 bits which are used to encode 256 possible intensity levels at the corresponding point on the photograph. Hence, without compression, transmission of the photograph requires that 8 million bits be sent over the communication link. A typical telephone line is capable of transmitting about 9600 bits per second; hence the picture transmission would require more than 10 minutes. Transmission times of this magnitude are unacceptable.
As a result, image compression systems are needed to reduce transmission time. It is also apparent to those skilled in the art that image compression systems may also be advantageously employed in image storage systems to reduce the amount of memory needed to store one or more images.
However, under a fixed compression system, an area of the image data may be distorted due to fixed compression rates. Specifically, the actual compression rates of different regions in the same image data are different because of different region contents. The actual compression rates of the regions with smooth contents are higher, and on the other hand, the actual compression rates of the regions with complex contents are lower. To achieve the best compression quality, an encoder can reserve some bits from first encountered smooth regions and utilize the saved bits for encoding latter complex regions. Nevertheless, one-pass encoding is normally adopted for encode processing for consistency and simplification. With one-pass encoding, image data passes through the encoder once, and the fixed compression rate is applied as the content is encountered. Thus, when a complex region is encountered before any smooth region is encountered, there are no saved bits for encoding the complex region and the complex region is encoded with the fixed compression rate. Consequently, it may cause distortion of the complex region of the image data.